In the past, electric fields have been used to break certain emulsions. This technology, for example, has been widely used in the oil refining industry where some water and salt is inadvertently added in the production and transportation of petroleum crude. Additional water is intentionally added in an emulsification stage at the refinery input. The water absorbs salt and other impurities from the crude oil, which are then separated with the water in an electrostatic coalescer.
In the electrostatic coalescence process, electric fields are applied in the coalescer, which induce electric dipole moments in the water phase. The induced dipoles create interparticle attraction, thereby accelerating coalescence of the water droplets. The water, with its load of impurities, is then separated by sedimentation.
Although electrostatic coalescence is inexpensive, its utilization is limited by a severe restriction on the strength of the electric field which can be used because of problems associated with electric power consumption in the conducting phase of the emulsion. Further, when the electric field strength reaches the level of a few thousand volts per centimeter, arcing occurs because of electrical breakdown, which presents insurmountable obstacles to the practical application of the technology. In some applications, the use of the electrostatic method is limited because of temperature increases associated with the power consumption. For example, electrostatic methods could not be used in processing of unstable compounds such as fermenter broth in biochemical applications because of the temperature increases to the system.
Additionally, the use of electrostatic fields is limited at both high and low water concentrations. For practical reasons, the electrodes of electrostatic coalescers are immersed in the emulsions. When the water content is in the 5-10% range, chains of coalescing water droplets can extend between the electrodes, thus shorting the device. To prevent this, electrically insulated electrodes are employed. This practice has severely limited the application of state-of-the-art coalescers because the development of insulated electrodes has proven to be difficult, especially when used in corrosive environments and on a large scale. At low water concentrations, e.g., 0.1%, the dispersed water phase droplets are widely separated so that it is impossible to induce sufficient electric moment to coalesce the water phase in practical applications.
Lastly, elevated temperatures are used to lower viscosity and improve the effect of electric fields in desalting crude oils by electrostatic methods. The cost of steam increases with the amount of salt in the crude and can be as much as one-half of the total cost of desalting. As a result of this and other inefficiencies associated with electrostatic coalescence, breaking an emulsion by that method requires long residence time, limiting the applicability of this method.
Several attempts have been made to apply magnetic fields to breaking emulsions. Fundamental differences in magnetic and electric properties of materials should allow many of the practical problems associated with use of electric fields to be overcome. For example, there is no magnetic analog to electric charge. Because of this, no energy dissipating conduction current is involved with the use of magnetic fields as is the case with the use of electric fields, and since only DC fields are employed in the magnetic case, there are no induced eddy currents. Furthermore, magnetic fields can be applied from outside of the emulsion. Because of this, electrodes are not needed. With use of magnetic fields, problems of energy dissipation and of "back" shielding of the emulsion by conduction currents and of use of insulated electrodes are completely removed. Further, since there is no fundamental limit to the level of magnetic field which can be applied, it is possible to develop magnetic energy densities in the emulsions which are much greater than is possible with electric fields. Because of this, the magnetic method does not require the use of elevated temperature which is a major cost factor in the electric method.
In the prior art, three distinguishing problems characterize earlier attempts to apply magnetic fields to demulsification; all these limitations have been overcome by the method of the present invention.
First, means such as Hubby, U.S. Pat. No. 3,412,002, employing Lorentz deflecting forces are impractical, and those, such as Roberts, U.S. Pat. No. 1,978,509, which do not teach the use of practical additives to impart strong magnetism to one phase of the emulsion, cannot be practiced.
Second, while others have proposed use of magnetic additives (Papell, U.S. Pat. No. 3,215,572; Cooper, U.S. Pat. No. 4,108,767; and Kaiser, U.S. Pat. Nos. 3,796,660 and 3,806,449), they appear to be unaware of the physicochemical aspects and consequences of coalescence in a magnetic field. For example, Kaiser writes of "spherical particles", whereas liquid magnetic particles which are most amenable to coalescence actually assume ellipsoidal shapes in a magnetic field. The criteria for preparing and choosing suitable ferrofluids stressed in the Kaiser patents has been found to be irrelevant. Kaiser bases the usefulness of ferrofluids on interfacial tension and spreading coefficients. Kaiser's concern is that the ferrofluid will serve to lower interfacial tension and actually stabilize the emulsion rather than allowing the magnetic field to break it. It has now been discovered in connection with the present invention that the use of surfactants which affect interfacial film stability enhances the effects of magnetic additives on coalescence in a magnetic field. The surfactants also lower interfacial tension but do not stabilize the emulsion. Some surfactants lower interfacial tension, but do not enhance coalescence in a magnetic field. They appear to have neither stabilizing nor destabilizing effects on emulsions. Without the proper interplay between film stability and magnetic field strength, the magnetized particles chain and flocculate and do not coalesce.
While chained and otherwise flocculated droplets will separate from the emulsion because of differences in density, such separation is undesirable because it also leads to a loss of the continuous phase which is carried out in the voidages between the flocculated or chained droplets. Chaining results in a disposal problem, to a loss of the expensive organic liquid component of the emulsion, and to practical limitations in the application of magnetic separators to demulsification.
Third, magnetic means previously indicated for breaking magnetic emulsions are not practical for general application, especially when the water content is high. Batch-operated High Gradient Magnetic Separators (HGMS) are sometimes suggested because of their capabilities in complete separation of fine-sized magnetic materials. While HGMS separators have this capability, it is also a limiting factor in breaking emulsions in which one of the phases is magnetic. The magnetic phase rapidly plugs the stainless steel mesh filter structure used in HGMS, making its use impractical when the content of the magnetic phase of the emulsion is much greater than about 0.5%. Indeed, Example VIII of Kaiser '449 makes clear that a batch process is contemplated. The present invention is not limited in this manner. The continuously operating method of the present invention can treat emulsions with any level of internal phase which is physically possible.
The present invention has overcome problems of chaining by discovering a new role for surfactants in destabilizing the droplet interfacial film in the presence of a magnetic field. This destabilization diminishes the tendency of droplets to flocculate and chain in a magnetic field and promotes coalescence by allowing practical levels of the magnetic field to compress and rupture the interfacial film in short time exposures to the field. Additionally, the invention teaches the use of a new approach to high gradient magnetic separation which is continuous in operation, which is capable of separating internal phase droplets in the micron size range in high throughput operation, which does not plug even when processing high internal phase emulsions which are strongly magnetic, and which achieves high levels of recovery of the continuous phase of the emulsion.
It would therefore be desirable to develop the magnetostatic coalescence method because it does not suffer from the limitations associated with the use of electric fields. Magnetostatic coalescence can be used in processing systems of a high electrical conductivity such as organic-liquid-in-water ("OL/W") emulsions. There are no induced electric currents or electric power consumption in the emulsion and no electrodes are employed. The magnetostatic coalescence method is also safer for use around flammable materials, such as crude oil and gasoline feedstocks. While heat is not developed in the emulsion during breaking, the method can be operated over any temperature range applicable to the emulsion.